Induction Motor Cogging and Crawling

Induction motors that cog or crawl will not accelerate
to full speed. Cogging motors do not accelerate at all, and crawling induction
motors stop accelerating at part speed. Acceleration can also be limited
by the torque output of the motor relative to the load torque at that
speed.

Cogging

Induction motors have a series of
slots in the stator and in the rotor. These slots should not be
equal in number because if they are, there is a good chance that
the motor will not start at all due to a characteristic known as
cogging. The slots will align like a stepper motor.
For this reason, there are an unequal number of slots in the rotor
and in the stator, but there can still be situations where the slot
frequencies coincide with harmonic frequencies and this can cause
torque modulations. The slots are skewed to keep an overlap on all
slots to reduce this problem.

Crawling

Another characteristic of induction motors, is crawling.
There are harmonic fluxes developed in the gap due to the magnetics of
the motor. These harmonics create additional torque fields. A common problem
is with the seventh harmonic where the seventh harmonic creates a forward
rotating torque field at one seventh of the synchronous speed. There will
be a maximum torque just below 1/7 Ns and if this is high enough, the
net torque can be higher than the torque due to the line frequency where
at 1/7 Ns, the slip is high. This can cause the motor to crawl at just
below 1/7 synchronous speed.
There is another crawl speed at 1/13 Ns.

Crawling with Soft Starters

When a motor is started with a soft starter, the gap harmonics are increased
by the harmonic currents produced by the phase controlled SCRs of the
soft starter.
A motor that has a tendency (small) to crawl when operated with a clean
supply, will have a much greater tendency to crawl when controlled by
SCRs and a chopped waveform.
The subject of harmonic torques and harmonic fluxes can get very complex
when you try to analyse the effects of all the electrical harmonics, plus
the magnetic harmonics plus the slot noise.
The seventh harmonic is the best known and documented crawl speed, but
others can exist due to the total interaction of all harmonic sources.
The two critical speeds are 1/13 Ns and 1/7 Ns. If the motor crawls just
below either of these speed, there is a definite interaction between the
motor and the harmonics produced by the soft starter. At other odd order
non triplen harmonic speeds, there can also be more complex interactions.
The design of the motor, no of slots in rotor and no of slots in stator,
and the skew of the slots are all selected to minimise these problems,
but all design is a compromise.

A motor crawling at say 1/7 Ns will produce maximum torque at a low slip
(relative to 1/7 Ns) in the same way that the maximum torque is produced
at just below Ns. There will therefore be a "slip" frequency
at the maximum torque. This will cause a modulation in the current flow
in the motor and this in turn can modulate the commutation angle of the
SCRs.
Where the SCRs are triggered relative to the voltage wave form, there
will be a modulation of the conduction angle of the SCRs as observed.
This will tend to accentuate the crawl torque by effectively amplitude
modulating the line frequency applied to the motor. Where the SCRs are
triggered relative to the current waveform, the phase modulation will
not affect the conduction angle of the SCRs and the effect will not affect
the crawl torque.

Where the phase modulation of the commutation angle causes an amplitude
modulation of the motor current, and there is a current control loop,
it is possible that the response time of the loop can further amplify
the modulation.
The greater the level of modulation, the lower the probability that the
motor will accelerate through the crawl speed.
In the same way that the torque just below crawl speed is a maximum, the
torque just above crawl speed is a minimum. There must be sufficient synchronous
torque at the minimum to accelerate the load to full speed.

The major influence of the supply on this scenario, is that an increased
supply impedance will result in increased conduction angles and reduced
harmonics which will reduce the crawl torque. A reduced supply impedance
will cause a reduced conduction angle for the same current and this will
increase the harmonic content and result in an increased crawl torque.

If you suspect that you may have a crawling situation, characterised by the motor not accelerating and strong vibrations,
the best indication is to determine the actual shaft speed where the motor
acceleration stops. If this is at just below 1/13 Ns (115RPM @ 50Hz or 138RPM @ 60Hz) or
1/7 Ns (214RPM @ 50Hz or 257RPM @ 60Hz) then I would suggest that this is a very likely
scenario.